Fuel cells are emerging devices for generating electrical energy primarily by converting hydrogen to water as the only reaction product. However, the availability of pure hydrogen as a fuel source is a significant limitation on uses of fuel cells since there is no present infrastructure for making hydrogen widely available. Consequently, fuel cells are being developed to directly use readily available and easily transportable fuels, such as methanol. The development of compact, portable devices powered by direct methanol fuel cells (DMFCs) requires a capability to efficiently use methanol fuel directly.
With polymer electrolyte membranes that are currently available, e.g., Nafion.RTM., there is significant methanol permeation through the membrane from the cell anode to the cathode. Excessive methanol permeation at typical rates equivalent to 100-200 mA/cm.sup.2 presents a major problem in direct methanol fuel cell (DMFC) systems. Such methanol "crossover" corresponds to severe fuel loss because each methanol molecule that crosses through the membrane to the cathode recombines directly with oxygen at the air cathode without producing electrical energy. Fuel utilization is defined by the ratio:
(cell current)/[(cell current)+(crossover current)]. At a crossover rate of 100-200 mA/cm.sup.2, fuel utilization of a DMFC would be as low as 50% at typical fuel cell operating conditions. As a comparison, a fuel utilization rate of around 99% has been achieved in hydrogen/air fuel cells, where gaseous hydrogen has a much lower permeability through the membrane.
The methanol permeate usually combines readily with oxygen (air) on Pt catalyst at the cathode to form water and carbon dioxide. This process likely occurs through the short circuit of methanol electrooxidation and oxygen electroreduction reactions. The oxygen cathode is thus further depolarized when consuming the methanol permeate and additional cathode losses result.
Furthermore, any unreacted methanol at the cell cathode adversely affects the oxygen reduction process: the methanol can wet the cathode structure, causing the cathode catalyst to be deactivated and the cathode backing to be flooded and become inaccessible to oxygen in the cathode feed stream. These adverse effects of methanol crossover are even more profound when a limited airflow at ambient air pressure has to be used to obtain the simplicity and light weight of a portable direct methanol fuel cell power device.
Methanol crossover rates can be reduced, in principle, by lowering the methanol feed rate. But such a reduction in methanol feed rate can result in maldistribution of methanol flow over the active surface area of the membrane, with concomitant mal-distribution of current density, particularly when conventional flow channel designs are used. Consequently, the anode feed flow rates typically employed are sufficient to limit the methanol concentration drop between the cell inlet and outlet to only 30%. One aspect of the present invention provides a uniform distribution of methanol over the active surface of the membrane.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.